As liquid crystal devices find wider application, there is motivation to develop new and easier ways to make or use packages of these special materials. It is now known that some liquid crystal materials and certain liquid polymer precursors may be mixed together, the mixture formed into a film, and the polymer precursor materials allowed to react and cure. The resulting product is a polymer film that contains dispersed or encapsulated therein, many very small drops of liquid crystal material. Depending upon the nature of the liquid crystal material, the film may be opaque at room temperature. However, when that film is heated, stressed, or subjected to an electrical or magnetic field across its thickness, those portions of the film tend to become transparent.
Dispersion of liquid crystal material in a cured polymer matrix film is a convenient package for working with the liquid crystals. There have been several methods proposed for forming these films, including thermal cure, ultraviolet cure, phase separation by cooling a thermoplastic-polymer/liquid crystal mixture, and evaporation of solvent from a thermoplastic/liquid crystal solution. However, there are shortcomings associated with each of these known methods for forming these polymer-dispersed liquid crystal films.
Generally, when using a thermal cure such as with thermoset materials which cure by cross-linking, the polymer precursors must be handled in two parts to avoid premature curing. In addition, commonly used liquid crystal materials usually cannot tolerate high temperatures. Therefore, the polymer precursors must be chosen to be curable at about room temperature. Further, once all ingredients are mixed, the cure begins and the cure is relatively slow which leads to handling problems and aging problems in the polymer film.
The use of ultraviolet curing methods is also not ideal, since this method requires the addition of photoinitiators to the film. These photoinitiators may result in shortened film life unless special, non-trivial protective steps are taken.
Lastly, there are problems associated with the use of the thermoplastic-based films for producing the final film. The thermoplastic-based films and their physical characteristics such as refractive index, are extremely temperature sensitive since they are not cross-linked. In addition, high temperature usage of these thermoplastic films are limited because of undue softening and melting of the film. Lastly, because of the nature of the way these films are prepared, essentially by freezing or evaporation of a solvent, the resulting films are typically characterized by non-uniform thickness and properties.
Therefore, it is desirable to provide a method for curing these polymer films having the liquid crystal materials dispersed throughout. It is further desirable that such method avoid and alleviate the shortcomings associated with the previous methods.